Genesi e significato del riempimento della depressione carsica di Piano Locce sul Gran Sasso d’Italia (Abruzzo)

Abstract

Magaldi D. et al., The filling of karstic depression of Piano Locce (Gran Sasso range, Abruzzi, Italy) genesis and significance. (IT ISSN 0394-3356, 2009).
A deep test hole down to 90 m was carried out on the clastic filling of a karstic valley (polje) of the Gran Sasso (Piano Locce) range
near S.Stefano di Sessanio (L’Aquila Province, Italy ). This allowed us to sample alternated lacustrine and pedogenetic materials almost
exclusively formed on medium-fine textured pyroclastic deposits.
On almost one hundred samples some routine chemical-physical, optical micro morphologic, SEM –EDS micro- chemical determination and spectrometric by X ray emission analysis were performed.
After the results elaboration, the collected samples were grouped (α, β‚ γ) as follows:
α Group was related to not pedogenetic sedimentary material (lake and or marsh);
β Group was identified as soil or soil derived material perhaps to be classified as Udifluvent according to USDA Soil Taxonomy;
γ Group was considered a mixture of the previous groups, except soils, which were classified as Hydraquents. The pedogenetic process was prevailing on the ripening, so originating anisotropic microstructures with some orientation of clay domains in thin sections of
undisturbed samples.
The SEM analysis indicated that most of volcanic glass was completely weathered: only the less weathered sample of the core bottom
(78-80 m) was classified as related to latite-trachyte rock. Such a composition could be related with the Vico Pleistocene eruption, which is constituted by trachite-phonolitic lavas ((PECCERILLO, 2005). Consequently, this sample could be correlated to the pyroclastic deposit of the Adriatic Sea collected from a core 170,000 years before the present date (CALANCHI et al., 2008).
Taking into consideration the low contents of Na2O and ClO , a correction (3 – 4 %) was made for the sum of alkaline oxides in order to
classify the glass as a tephritic-phonolite and phonolite lavas which are assumed to be erupted by the Campania volcanism
(PECCERILLO, 2005). Based on the composition of both pyroxene and plagioclase minerals and the paragenesis, 2 samples were attributed to the Latium volcanism.
It was also considered that the silt/clay ratio vs. the core depth, which demonstrated an inverse trend with the percentage of organic
matter, could be assumed as a paleo-climatic index. Low values of the index could indicate more favourable conditions for pedogenesis unlike higher values which could indicate colder and perhaps more arid periods. In conclusion, all results likely indicate that the Piano Locce depression during Quaternary age was subjected to various sedimentary and pedogenetic phases which occurred on pyroclastic deposits coming from the Co-magmatic Roman Province, starting from the present age to more than 200,000 years ago.
Climatic variations in a high mountain environment are suggested by trends of both organic matter and silt/ clay ratio as well as micromorphological features, which in very few cases show periglacial aspects. Some scarcely developed hydromorphic soils only formed when the favourable climatic conditions occurred. A very limited colluvial and/ or eolian material from more developed soils of different climatic environments (red soils such as Xeralfs or Ustalfs) together with some occasional deposition of creeks from surrounding carbonate slopes contributed filling the Piano Locce and nearby karst valleys.